Gary, Although I don't know the particulars (check sites on electric vehicles), you can use the diode and large capacitors to capture the energy. A good friend of mine used this technique with a three phase motor used in a energy efficient drive system. He had size constraints too, so used arrays of high capacitance ceramic capacitors for the energy storage. The energy was used to supply a dc/dc converter used for, among other things, the 3-phase generator/ controller electronics. It would be interesting to use the second set of MOSFETs as synchronous rectifiers which turn on as the main set turns off. You would still need diodes to handle the crossover time so that no cross conduction occurred, but the internal diode may serve that purpose. And, yes, you might need a floating gate drive. Have fun. Ed -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Gary Neal Sent: Tuesday, October 09, 2001 10:55 AM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [PIC]: Mosfet speed control of 12V DC motor - Partially Solved OK, let me get this straight. I could use the same type of MOSFETS I am currently using for the drive side as the flyback device. Just use the internal diode in the MOSFETS? Would I just connect the gate straight to ground? I've got extra MOSFETS lying around and I've got extra room on my heat sink. Should be able to just wire them up and try it out. That would save me an order to Digikey/Newark :-) IRF says the IRLZ44N can handle a "Continuous Source Current (Body Diode)" of 47A. Does that mean the internal diode in this thing can handle up to 47A (with proper heat sinking)? I'm a little confused about the efficiency and corresponding output power of the entire system with this flyback diode in there now. This is a portable application, so efficiency and power are a concern (lesser of a concern than learning stuff and making it run though). Anyway, the way I see it, if I use a MOSFET for the flyback diode, there's ~1.3V drop across it's diode. That means more heat dissipated in the MOSFET. If I use a Schottkey diode, ~.6V drop across diode, less heat dissipation in diode, but that extra energy that's not dissipated in the diode has to go somewhere right? Does that mean that the motor would correspondingly see more reverse current and heat up more (and also not put out as much power)? Does it mean that the overall system efficiency is "about" the same no matter what device I use in there? Gary At 11:13 AM 10/9/01 -0400, Douglas Butler wrote: >The flyback diodes shouldn't need to be any bigger (or more expensive) >than the MOSFETS you are already using. For convienient mounting (at >the expense of cost) you could use another of the same MOSFET, using its >inherent reverse diode as your flyback diode. Real diodes should be >cheaper, and Schottkeys would run cooler. > >Sherpa Doug > > > -----Original Message----- > > From: Spehro Pefhany [mailto:speff@INTERLOG.COM] > > Sent: Tuesday, October 09, 2001 10:50 AM > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Re: [PIC]: Mosfet speed control of 12V DC motor - Partially > > Solved > > > > > > At 08:57 AM 10/9/01 -0400, you wrote: > > >Guys, > > > > > > Thanks for the help. Flyback Diodes! Daaaa! > > Completely forgot > > >that. I put a 1N4004 (only 1A, but the biggest one I had > > lying around) on > > >the motor and it worked like a charm. I'm running at about > > 20khz. This > > >diode only gets hot when the duty cycle is less than about 80%. > > > > It needs to be MUCH bigger, and MUCH faster. A 1N4004 is not > > much use at > > 20kHz, you'll be putting huge current pulses through your > > MOSFETs during > > the LONG reverse recovery of these molasses-like antediluvian > > rectifiers. > > You can also use a similar number of parallel MOSFETs, which > > may have less > > power dissipation than the diode, but will be more costly. > > > > > I don't have a whole lot of experience sizing these flyback > > >diodes. Someone said it should be rated for as much as the > > full current of > > >the motor (~30A). A 30A diode is HUGE! These big diodes > > are all Stud > > >mount types it seems. This doesn't seem very compatible to > > mounting to the > > >leads of a motor. > > > > They don't have to go right on the leads, right by the > > MOSFETs is fine. > > You'll need a heat sink. The wires will radiate RFI in any case. > > > > > Can anyone give me some insight on what to use? > > Potential sources > > >for the product? I'd like to keep it as cheap as possible. > > Don't want to > > >spend $20 on a single diode when I've only got about a total > > of $10 in all > > >the other parts combined. > > > > Look at: http://www.onsemi.com/pub/Collateral/MBR3045WT-D.PDF > > > > How about an ONsemi or IR MBR3045WT 30A, 45V Vf=0.60V. in > > TO-247 case for > > only $1.10 in 100s (USD). Worst case is probably around 50% > > (15A average), > > but I have not worked it out in detail, if that's right you'll need to > > get rid of around 11W. Parallel the two diodes in the '3045. > > > > > Also, any other methods to help this reverse > > current from the > > >motor? Slower PWM frequency (audible noise doesn't really matter to > > >me)? Some configuration of caps? I'm all ears when it comes to you > > >experts :-) > > > > No, you need to direct this current through the motor. It's what you > > need to maintain decent speed control (it's like applying a variable > > DC voltage to the motor). You can slow down the PWM and > > you'll get more > > noise and less heat loss in the MOSFETs (maybe not all that > > significant) > > but you'll still need a diode or an equivalent active device, and it > > will get about as hot. > > > > Best regards, > > > > Spehro Pefhany --"it's the network..." "The > > Journey is the reward" > > speff@interlog.com Info for manufacturers: >http://www.trexon.com >Embedded software/hardware/analog Info for designers: >http://www.speff.com > /.-.\ > (( * )) > \\ // Please help if you can: > \\\ http://dailynews.yahoo.com/fc/US/Emergency_Information/ > //\\\ > /// \\\ > \/ \/ > >-- >http://www.piclist.com hint: PICList Posts must start with ONE topic: >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > >-- >http://www.piclist.com hint: PICList Posts must start with ONE topic: >[PIC]:,[SX]:,[AVR]: ->uP ONLY! 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